A recording medium such as a magnetic tape in a VTR or a video floppy disk used for recording a video signal is rotated by a motor relative to a video head. For the purpose of motor phase control, a vertical synchronizing signal contained in the video signal is used as a reference signal phase which is compared in a phase system with a comparative signal phase provided by a phase generator (PG) signal indicative of the phase of one complete rotation of the motor.
On the other hand, in the case of motor phase control in an electronic still camera, quick start-up is demanded for a motor in view of the specific function of the camera. However, the motor phase control based on the phase comparison between the vertical synchronizing signal and the PG signal cannot satisfy the demand. This is because, when the NTSC signal having the field frequency of 60 Hz is used for the phase comparison, a period of time as long as several seconds is required until phase pull-in is completed.
Therefore, it has been a prior art practice that a high-speed signal generated from the synchronizing signal generator (SSG) is the image pickup system of the electronic still camera is used as a reference phase, while the FG signal indicative of the rotation speed of the motor is used as a comparative signal phase. Further, these signal phases are compared at the frequency higher than that of the PG signal, thereby completing the phase pull-in within a short period of time.
However, in the case of the above manner of phase control, the phase of the vertical synchronizing signal does not match that of the PG signal. That is, the mechanical position of the recording medium and the position of the vertical synchronizing signal on the magnetic pattern do have not a predetermined relation between them.
Therefore, it is now a common practice that, after the motor is servo locked, only the PG signal indicative of one complete rotation of the motor as well as the rotation phase of the magnetic sheet is used to reset the video synchronizing signal from the SSG, and the phase difference between the PG signal and the leading edge of the vertical synchronizing signal is set at 7H (H: one horizontal scanning period).
By the above manner of phase control, the phase pull-in is completed within several hundred milliseconds. Therefore, the period of the time required for the electronic still camera for recording an object could be shortened, and the possibility of losing the shutter chance by quick depression of the release switch of the camera could be minimized.
However, the factor which actually limits the length of time required until video recording is enable in the electronic still camera system is the period of time elapsed until the PG signal and the vertical synchronizing signal are stably and positively synchronized. Because the period of time required until the motor is placed in its servo locked condition occupies the greater part of the above period of time, it is further demanded to place the motor in the servo locked condition as quick as possible without any fluctuation in the period of time.
Among a plurality of methods for deciding the servo locked condition of the motor which provides the basis for resetting the SSG, there are the following two typical methods:
1. The method in which the phase error voltage is continuously detected so as to decide that the motor is servo locked as soon as the phase error attains a level within a predetermined range.
2. The method in which the decision for the servo locked condition of the motor is not especially made, and the motor is presumed to be servo locked when a certain predetermined period of time has elapsed after the motor is started.
However, although the first method can positively decide that the motor is completely servo locked, the first method has such an essential disadvantage that the period of time elapsed, until the motor is servo locked, tends to fluctuate. This tendency adversely effects the decision.
On the other hand, in the case of the second method, the period of time elapsed until the motor is decided to be servo locked is freed from fluctuation. However, in the second method, it is necessary to take into consideration various kinds of other sources of fluctuation encountered until the motor is actually servo locked, and it is also necessary to take into consideration a margin for compensation of the fact that the servo locked condition of the motor is not directly detected. In view of the above necessity, the second method has such a disadvantage that a considerably long period of time must be prepared before the SSG is reset.
Thus, each of the first and second methods can satisfactorily work when the fluctuation of the period of time required until the motor is servo locked is made uniform, and the phase is pulled in within a short period of time.
In order that the phase pull-in can be achieved within a short period of time, it is generally necessary to widen the operation bandwidth of the phase system in the servo system, that is, to widen the f (frequency) characteristic of the servo system.
However, when the f characteristic of the servo system is widened, the servo system may respond to internal noise, disturbance, etc. Therefore, the f characteristic of the servo system cannot be excessively widened. Especially, when the f characteristic is set so as not to respond to an "encoder FG error" which appears once for every one complete rotation of the motor, the step response time is determined by such an f characteristic, so that the period of time required until the motor is servo locked cannot be shortened beyond a certain limit. Thus, there is such a disadvantage that maximum fluctuation may occur finally in the period of time required until the motor is servo locked.